Not Applicable
Not Applicable
(1) Field of the Invention
The present invention relates to a process for treating surfaces of substrates using ultraviolet light along with cooling of the surface to produce surface activation. In particular, the present invention relates to a preferred process for pretreating surfaces of substrates by providing water to provide the cooling of the surface between the radiation and the substrate. The process is particularly useful when ozone is used to treat the surface, particularly with the water, since ozone is six times more soluble in water than oxygen. The treatment enhances surface activation, allows for surface modification in shorter time periods and preferably increases the wetting characteristics.
(2) Description of the Related Art
Manufactured surfaces of substrates always contain undesirable compounds or additives that limit or reduce adhesion to an adhesive or paint film. Hence, surface preparation, which includes cleaning and activation of the surfaces, of polymeric, polymer composite or metal substrates is carried out prior to applying protective paint films or adhesive bonding. Surface preparation determines the mechanical and durability characteristics of the composite created. Currently the techniques used for surface preparation are mechanical surface treatments (e.g. abrasion), solvent wash and chemical modification techniques like corona, plasma, flame treatment and acid etching. Each of the existing processes have shortcomings and thus, they are of limited use. Abrasion techniques are found to be time consuming, labor intensive and have the potential to damage the adherent surface. Use of organic solvents results in volatile organic chemical (VOC) emissions. Chemical techniques are costly and are of limited use with regard to treating three dimensional parts, can be a batch process (such as plasma, acid etching) and need tight control.
The use of lasers for surface treatment is known in the art. The focussed beams of the lasers make it difficult to treat a large surface. U.S. Pat. No. 4,803,021 to Werth et al describes such a method. U.S. Pat. No. 4,756,765 to Woodroffe describes paint removal with surface treatment using a laser.
Plasma treatment of surfaces is known in the art. Relatively expensive equipment is necessary for such treatments and plasmas are difficult to control. The surfaces are treated with vaporized water in the plasma. Illustrative of this art are U.S. Pat. No. 4,717,516 to Isaka et al., U.S. Pat. No. 5,019,210 to Chou et al., and U.S. Pat. No. 5,357,005 to Buchwalter et al.
A light based process which cleans a substrate surface also creates a beneficial chemistry on the surface for adhesive bonding and paintability is described in U.S. Pat. No. 5,512,123 to Cates et al. The process involves exposing the desired substrate surface to be treated to flashlamp radiation having a wavelength of 160 to 5000 nanometers. Ozone is used with the light to increase the wettability of the surface of the substrate being treated. Surfaces of substrates such as metals, polymers, polymer composites are cleaned by exposure to the flashlamp radiation. The problem with the Cates et al process is that the surface of the substrate is heated to a relatively high temperature, particularly by radiation above 500 nanometers and relatively long treatment times. Related patents to Cates et al are U.S. Pat. No. 3,890,176 to Bolon, U.S. Pat. No. 4,810,434 to Caines; U.S. Pat. No. 4,867,796 to Asmus et al; U.S. Pat. No. 5,281,798 to Hamm et al and U.S. Pat. No. 5,500,459 to Hagemever et al and U.K. Pat. No. 723,631 to British Cellophane. Non-patent references are: Bolon et al., xe2x80x9cUltraviolet Depolymerization of Photoresist Polymersxe2x80x9d, Polymer Engineering and Science, Vol. 12 pages 109-111 (1972). M. J. Walzak et al., xe2x80x9cUV and Ozone Treatment of Polypropylene and poly(ethylene terephthalate)xe2x80x9d, In: Polymer Surface Modification: Relevance to Adhesion, K. L. Mittal (Editor), 253-272 (1995); M. Strobel et al., xe2x80x9cA Comparison of gas-phase methods of modifying polymer surfacesxe2x80x9d, Journal of Adhesion Science and Technology, 365-383 (1995); N. Dontula et al., xe2x80x9cA study of polymer surface modification using ultraviolet radiationxe2x80x9d, Proceedings of 20th Annual Adhesion Society Meeting, Hilton Head, SC (1997); C. L. Weitzsacker et al., xe2x80x9cUtilizing X-ray photoelectron spectroscopy to investigate modified polymer surfacesxe2x80x9d, Proceedings of 20th Annual Adhesion Society Meeting, Hilton Head, S.C. (1997); N. Dontula et al., xe2x80x9cUltraviolet light as an adhesive bonding surface pretreatment for polymers and polymer compositesxe2x80x9d, Proceedings of ACCE""97, Detroit, Mich.; C. L. Weitzsacker et al., xe2x80x9cSurface pretreatment of plastics and polymer composites using ultraviolet lightxe2x80x9d, Proceedings of ACT""97, Detroit, Mich.; N. Dontula et al., xe2x80x9cSurface activation of polymers using ultraviolet activationxe2x80x9d, Proceedings of Society of Plastics Engineers ANTEC""97, Toronto, Canada. Haack, L. P., et al., 22nd Adhesion Soc. Meeting (Feb. 22-24, 1999).
Non-pulsed UV lamps have been used by the prior art. These are described in: xe2x80x9cExperimental Methods in Photochemistryxe2x80x9d, Chapter 7, pages 686-705 (1982). U.S. Pat. No. 5,098,618 to Zelez is illustrative of the use of these types of lamps.
A disadvantage of the ultraviolet lamp treatments of the prior art is that they are time consuming and sometimes unreliable. To achieve suitable surface chemistries for adhesive bonding and painting purposes, exposure times for certain materials like polypropylene, thermoplastic olefins (TPO""s) tend to be of the order of 5 to 60 minutes. In many cases there is a limit on the length of time to which one may expose the substrates to UV since there is a fear of degrading the substrate. There is a need for development of an environmentally friendly as well as cost effective and robust surface treatment process which can be used over a range of surfaces.
It is therefore an object of the present invention to provide process and apparatus which are reliable and which can produce a highly active surface. It is further an object of the present invention to provide process and apparatus which are economical. These and other objects will become increasingly apparent by reference to the following description and the drawings.
The present invention relates to an improved method for modifying a surface of a substrate by generating optical energy having ultraviolet wavelengths and irradiating the surface of the substrate, preferably comprising a polymer, with the optical energy which comprises: irradiating the surface with the optical energy to modify the surface, wherein the wavelengths are between about 160 nanometers and 500 nanometers without higher wavelengths with cooling of at least the surface of the substrate. The cooling of the surface is to temperatures less than 100xc2x0 C., preferably between 0 and 50xc2x0 C.
The present invention also relates to a preferred improved method for modifying a surface of a substrate, particularly a polymer surface, by generating optical energy and irradiating the surface of the substrate with the optical energy with an optical power density of sufficient intensity to modify the surface, which comprises: providing water on the surface to be modified prior to irradiating the surface to modify the surface and then irradiating the surface with the optical energy to modify the surface, wherein the wavelengths are between about 185 nanometers and 254 nanometers.
The present invention further relates to an apparatus for modifying a surface of a substrate comprising: an optical energy source for generating optical energy having ultraviolet wavelength components ranging from about 160 to 500 nanometers without higher wavelengths; a cooling means for cooling at least the surface of the substrate; and an electrical power supply operably coupled to power the energy source.
The present invention also relates to a preferred improved method for modifying a surface by irradiating the surface with optical energy at an intensity sufficient to modify the surface forming the surface, which comprises: providing water on the surface to be modified prior to irradiating the surface and then irradiating the surface with the optical energy to photodecompose organic materials present on the surface, preferably with ozone dissolved in the water, wherein the wavelengths are between about 160 nanometers and 500 nanometers without higher wavelengths.
The present invention further relates to a preferred improved method for modifying a surface by irradiating the surface with pulsed optical energy at an intensity sufficient to modify the metallic surface, which comprises: providing water on the surface to be modified prior to irradiating the surface and then irradiating the surface with the optical energy to modify any organic materials present on the surface, preferably with ozone dissolved in the water, wherein the wavelengths are between about 185 nanometers and 254 nanometers without higher wavelengths.
The present invention relates to an apparatus for modifying a surface of a substrate comprising: an optical energy source for generating optical energy having ultraviolet wavelength components ranging from about 160 to 500 nanometers without higher wavelengths; an electrical power supply operably coupled to power the energy source; and means for providing water on the surface for modifying the surface of the substrate when irradiated.
In the present invention it was discovered that if higher wavelengths of light above about 500 nm are used, a thermoplastic surface of the substrate is heated to above the glass transition temperature. This allows the newly formed chemically active groups to rotate into the melted surface thus, decreasing the activity of the surface. By limiting the wavelengths produced by the UV lamp or by the use of filters, the light above 500 nanometers can be largely eliminated with the cooling.
It is also necessary to cool the surface to freeze the chemically active groups to the outside of the substrate. This is a very unexpected finding, since the prior art is concerned only with heating the surface. The cooling can be by a cooling gas flushed over the surface and/or by using short durations (3 minutes or less) of the ultraviolet light with liquid cooling.
The process of the present invention is cost effective for pretreatment of surfaces of polymers, polymer composites and metals prior to adhesive bonding or painting. The process creates beneficial surface chemistries for adhesive bonding or painting. The advantages of this process over the existing prior art is that the process is cheaper than chemical modification techniques such as plasma and is not a batch process as with plasma and acid etching. Further, the new process can be used to treat three dimensional parts where corona and flame treatments have difficulty in treating. The process can also be used to treat large surface areas quicker than flame treatment. In addition, the substrates are not as sensitive to degradation as when exposed to flame, such as with flame treatment. The process is environmentally friendly as compared to solvent wash, acid etching and mechanical abrasion techniques. The process is much cheaper than processes using UV exciter lasers which are cost intensive and work on the principle of ablating the surface layers or roughening the surface or amorphizing the top surface layers. In comparison to the existing ultraviolet lamp techniques, the current process reduces the process times for treating various substrate surfaces (thus making it cheaper, and avoiding degradation of the substrates) and achieves surface modifications which were not possible. This invention can be used to tailor the chemistry of the substrate surface by using other cooling reactivating vapors (ozone, ammonium and nitrogen) in between the substrate surface and ultraviolet light.
The process of the present invention uses one step for cleaning the surface and functionalizing the surface so that the materials can be painted without the use of adhesion primers or the use of costly and environmentally unfriendly treatment steps. Further, this process can be used prior to adhesive bonding of polymers, polymer composites and metals, to achieve good bond strengths.
The process can be used for treating carbon fibers and carbon whiskers prior to their use in composites, for instance. An alternate use of the process is in the packaging industry where polymers are printed with ink. This process is an easier, flexible or alternate way for them to create materials with different barrier properties for solvents.